Detecting multitouch events in an optical touch-sensitive device using touch event templates

ABSTRACT

An optical touch-sensitive device is able to determine the locations of multiple simultaneous touch events. The optical touch-sensitive device includes at least one emitter and detectors. Each emitter produces optical beams which are received by the detectors. Touch events disturb the optical beams. Touch event templates are used to determine the actual touch events based on which optical beams have been disturbed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/744,349 “Detecting Multitouch Events in an Optical Touch-SensitiveDevice Using Touch Event Templates” filed Jun. 19, 2015, which is acontinuation of U.S. application Ser. No. 13/460,703 “DetectingMultitouch Events in an Optical Touch-Sensitive Device Using Touch EventTemplates” filed on Apr. 30, 2012, now allowed, which is a continuationin part of U.S. application Ser. No. 13/059,817 “Method and Apparatusfor Detecting a Multitouch Event in an Optical Touch-Sensitive Device”filed on Feb. 18, 2011; now U.S. Pat. No. 8,350,831, issued on Jan. 8,2013, which is the U.S. National Phase application under 35 U.S.C. 371of International Application No. PCT/EP2009/005736 “Method and Apparatusfor Detecting a Multitouch Event in an Optical Touch-Sensitive Device”filed on Aug. 7, 2009; which claims priority to IE applicationS2008/0652 filed Aug. 7, 2008 and to IE application S2008/0827 filedOct. 10, 2008.

U.S. application Ser. No. 13/460,703 is also a continuation in part ofU.S. application Ser. No. 13/059,772 “Optical Control System WithModulated Emitters” filed on Feb. 18, 2011; now U.S. Pat. No. 8,227,742,issued on Jul. 24, 2012, which is the U.S. National Phase applicationunder 35 U.S.C. 371 of International Application No. PCT/EP2009/05739“Optical Control System With Modulated Emitters” filed on Aug. 7, 2009;which claims priority to IE application S2008/0651 filed on Aug. 7,2008.

The contents of these applications are incorporated by reference hereinin their entirety.

BACKGROUND

1. Field of Art

This invention generally relates to detecting touch events in atouch-sensitive device, especially optical approaches capable ofdetecting multitouch events.

2. Description of the Related Art

Touch-sensitive displays for interacting with computing devices arebecoming more common. A number of different technologies exist forimplementing touch-sensitive displays and other touch-sensitive devices.Examples of these techniques include, for example, resistive touchscreens, surface acoustic wave touch screens, capacitive touch screensand certain types of optical touch screens.

However, many of these approaches currently suffer from drawbacks. Forexample, some technologies may function well for small sized displays,as used in many modern mobile phones, but do not scale well to largerscreen sizes as in displays used with laptop or even desktop computers.For technologies that require a specially processed surface or the useof special elements in the surface, increasing the screen size by alinear factor of N means that the special processing must be scaled tohandle the N² larger area of the screen or that N² times as many specialelements are required. This can result in unacceptably low yields orprohibitively high costs.

Another drawback for some technologies is their inability or difficultyin handling multitouch events. A multitouch event occurs when multipletouch events occur simultaneously. This can introduce ambiguities in theraw detected signals, which then must be resolved. Importantly, theambiguities must be resolved in a speedy and computationally efficientmanner. If too slow, then the technology will not be able to deliver thetouch sampling rate required by the system. If too computationallyintensive, then this will drive up the cost and power consumption of thetechnology.

Another drawback is that technologies may not be able to meet increasingresolution demands. Assume that the touch-sensitive surface isrectangular with length and width dimensions L×W. Further assume that anapplication requires that touch points be located with an accuracy of δland δw, respectively. The effective required resolution is then R=(LW)/(δl δw). We will express R as the effective number of touch points.As technology progresses, the numerator in R generally will increase andthe denominator generally will decrease, thus leading to an overallincreasing trend for the required touch resolution R.

Thus, there is a need for improved touch-sensitive systems.

SUMMARY

An optical touch-sensitive device is able to determine the locations ofmultiple simultaneous touch events. The optical touch-sensitive deviceincludes at least one emitter and detectors. Each emitter producesoptical beams which are received by the detectors. The optical beamspreferably are multiplexed in a manner so that many optical beams can bereceived by a detector simultaneously. Touch events disturb the opticalbeams. Touch event templates are used to determine the actual touchevents based on which optical beams have been disturbed.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present invention will now be described, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is a diagram of an optical touch-sensitive device, according toone embodiment.

FIG. 2 is a flow diagram for determining the locations of touch events,according to one embodiment.

FIGS. 3A-3F illustrate different mechanisms for a touch interaction withan optical beam.

FIG. 4 are graphs of binary and analog touch interactions.

FIGS. 5A-5C are top views of differently shaped beam footprints.

FIGS. 6A-6B are top views illustrating a touch point travelling througha narrow beam and a wide beam, respectively.

FIG. 7 are graphs of the binary and analog responses for the narrow andwide beams of FIG. 6.

FIGS. 8A-8B are top views illustrating active area coverage by emitters.

FIGS. 8C-8D are top views illustrating active area coverage bydetectors.

FIG. 8E is a top view illustrating alternating emitters and detectors.

FIGS. 9A-9C are top views illustrating beam patterns interrupted by atouch point, from the viewpoint of different beam terminals.

FIG. 9D is a top view illustrating estimation of the touch point, basedon the interrupted beams of FIGS. 9A-9C and the line images of FIGS.10A-10C.

FIGS. 10A-10C are graphs of line images corresponding to the cases shownin FIGS. 9A-9C.

FIG. 11A is a top view illustrating a touch point travelling through twoadjacent wide beams.

FIG. 11B are graphs of the analog responses for the two wide beams ofFIG. 11A.

FIG. 11C is a top view illustrating a touch point travelling throughmany adjacent narrow beams.

FIGS. 12A-12E are top views of beam paths illustrating templates fortouch events.

FIG. 13 is a flow diagram of a multi-pass method for determining touchlocations.

DETAILED DESCRIPTION I. Introduction

A. Device Overview

FIG. 1 is a diagram of an optical touch-sensitive device 100, accordingto one embodiment. The optical touch-sensitive device 100 includes acontroller 110, emitter/detector drive circuits 120, and atouch-sensitive surface assembly 130. The surface assembly 130 includesa surface 131 over which touch events are to be detected. Forconvenience, the area defined by surface 131 may sometimes be referredto as the active area or active surface, even though the surface itselfmay be an entirely passive structure. The assembly 130 also includesemitters and detectors arranged along the periphery of the activesurface 131. In this example, there are J emitters labeled as Ea-EJ andK detectors labeled as D1-DK. The device also includes a touch eventprocessor 140, which may be implemented as part of the controller 110 orseparately as shown in FIG. 1. A standardized API may be used tocommunicate with the touch event processor 140, for example between thetouch event processor 140 and controller 110, or between the touch eventprocessor 140 and other devices connected to the touch event processor.

The emitter/detector drive circuits 120 serve as an interface betweenthe controller 110 and the emitters Ej and detectors Dk. The emittersproduce optical “beams” which are received by the detectors. Preferably,the light produced by one emitter is received by more than one detector,and each detector receives light from more than one emitter. Forconvenience, “beam” will refer to the light from one emitter to onedetector, even though it may be part of a large fan of light that goesto many detectors rather than a separate beam. The beam from emitter Ejto detector Dk will be referred to as beam jk. FIG. 1 expressly labelsbeams a1, a2, a3, e1 and eK as examples. Touches within the active area131 will disturb certain beams, thus changing what is received at thedetectors Dk. Data about these changes is communicated to the touchevent processor 140, which analyzes the data to determine thelocation(s) (and times) of touch events on surface 131.

One advantage of an optical approach as shown in FIG. 1 is that thisapproach scales well to larger screen sizes. Since the emitters anddetectors are positioned around the periphery, increasing the screensize by a linear factor of N means that the periphery also scales by afactor of N rather than N².

B. Process Overview

FIG. 2 is a flow diagram for determining the locations of touch events,according to one embodiment. This process will be illustrated using thedevice of FIG. 1. The process 200 is roughly divided into two phases,which will be referred to as a physical phase 210 and a processing phase220. Conceptually, the dividing line between the two phases is a set oftransmission coefficients Tjk.

The transmission coefficient Tjk is the transmittance of the opticalbeam from emitter j to detector k, compared to what would have beentransmitted if there was no touch event interacting with the opticalbeam. In the following examples, we will use a scale of 0 (fully blockedbeam) to 1 (fully transmitted beam). Thus, a beam jk that is undisturbedby a touch event has Tjk=1. A beam jk that is fully blocked by a touchevent has a Tjk=0. A beam jk that is partially blocked or attenuated bya touch event has 0<Tjk<1. It is possible for Tjk>1, for exampledepending on the nature of the touch interaction or in cases where lightis deflected or scattered to detectors k that it normally would notreach.

The use of this specific measure is purely an example. Other measurescan be used. In particular, since we are most interested in interruptedbeams, an inverse measure such as (1−Tjk) may be used since it isnormally 0. Other examples include measures of absorption, attenuation,reflection or scattering. In addition, although FIG. 2 is explainedusing Tjk as the dividing line between the physical phase 210 and theprocessing phase 220, it is not required that Tjk be expresslycalculated. Nor is a clear division between the physical phase 210 andprocessing phase 220 required.

Returning to FIG. 2, the physical phase 210 is the process ofdetermining the Tjk from the physical setup. The processing phase 220determines the touch events from the Tjk. The model shown in FIG. 2 isconceptually useful because it somewhat separates the physical setup andunderlying physical mechanisms from the subsequent processing.

For example, the physical phase 210 produces transmission coefficientsTjk. Many different physical designs for the touch-sensitive surfaceassembly 130 are possible, and different design tradeoffs will beconsidered depending on the end application. For example, the emittersand detectors may be narrower or wider, narrower angle or wider angle,various wavelengths, various powers, coherent or not, etc. As anotherexample, different types of multiplexing may be used to allow beams frommultiple emitters to be received by each detector. Several of thesephysical setups and manners of operation are described below, primarilyin Section II.

The interior of block 210 shows one possible implementation of process210. In this example, emitters transmit 212 beams to multiple detectors.Some of the beams travelling across the touch-sensitive surface aredisturbed by touch events. The detectors receive 214 the beams from theemitters in a multiplexed optical form. The received beams arede-multiplexed 216 to distinguish individual beams jk from each other.Transmission coefficients Tjk for each individual beam jk are thendetermined 218.

The processing phase 220 can also be implemented in many different ways.Candidate touch points, line imaging, location interpolation, touchevent templates and multi-pass approaches are all examples of techniquesthat may be used as part of the processing phase 220. Several of theseare described below, primarily in Section III.

II. Physical Set-Up

The touch-sensitive device 100 may be implemented in a number ofdifferent ways. The following are some examples of design variations.

A. Electronics

With respect to electronic aspects, note that FIG. 1 is exemplary andfunctional in nature. Functions from different boxes in FIG. 1 can beimplemented together in the same component.

For example, the controller 110 and touch event processor 140 may beimplemented as hardware, software or a combination of the two. They mayalso be implemented together (e.g., as an SoC with code running on aprocessor in the SoC) or separately (e.g., the controller as part of anASIC, and the touch event processor as software running on a separateprocessor chip that communicates with the ASIC). Example implementationsinclude dedicated hardware (e.g., ASIC or programmed field programmablegate array (FPGA)), and microprocessor or microcontroller (eitherembedded or standalone) running software code (including firmware).Software implementations can be modified after manufacturing by updatingthe software.

The emitter/detector drive circuits 120 serve as an interface betweenthe controller 110 and the emitters and detectors. In oneimplementation, the interface to the controller 110 is at least partlydigital in nature. With respect to emitters, the controller 110 may sendcommands controlling the operation of the emitters. These commands maybe instructions, for example a sequence of bits which mean to takecertain actions: start/stop transmission of beams, change to a certainpattern or sequence of beams, adjust power, power up/power downcircuits. They may also be simpler signals, for example a “beam enablesignal,” where the emitters transmit beams when the beam enable signalis high and do not transmit when the beam enable signal is low.

The circuits 120 convert the received instructions into physical signalsthat drive the emitters. For example, circuit 120 might include somedigital logic coupled to digital to analog converters, in order toconvert received digital instructions into drive currents for theemitters. The circuit 120 might also include other circuitry used tooperate the emitters: modulators to impress electrical modulations ontothe optical beams (or onto the electrical signals driving the emitters),control loops and analog feedback from the emitters, for example. Theemitters may also send information to the controller, for exampleproviding signals that report on their current status.

With respect to the detectors, the controller 110 may also send commandscontrolling the operation of the detectors, and the detectors may returnsignals to the controller. The detectors also transmit information aboutthe beams received by the detectors. For example, the circuits 120 mayreceive raw or amplified analog signals from the detectors. The circuitsthen may condition these signals (e.g., noise suppression), convert themfrom analog to digital form, and perhaps also apply some digitalprocessing (e.g., demodulation).

B. Touch Interactions

FIGS. 3A-3F illustrate different mechanisms for a touch interaction withan optical beam. FIG. 3A illustrates a mechanism based on frustratedtotal internal reflection (TIR). The optical beam, shown as a dashedline, travels from emitter E to detector D through an opticallytransparent planar waveguide 302. The beam is confined to the waveguide302 by total internal reflection. The waveguide may be constructed ofplastic or glass, for example. An object 304, such as a finger orstylus, coming into contact with the transparent waveguide 302, has ahigher refractive index than the air normally surrounding the waveguide.Over the area of contact, the increase in the refractive index due tothe object disturbs the total internal reflection of the beam within thewaveguide. The disruption of total internal reflection increases thelight leakage from the waveguide, attenuating any beams passing throughthe contact area. Correspondingly, removal of the object 304 will stopthe attenuation of the beams passing through. Attenuation of the beamspassing through the touch point will result in less power at thedetectors, from which the reduced transmission coefficients Tjk can becalculated.

FIG. 3B illustrates a mechanism based on beam blockage. Emitters producebeams which are in close proximity to a surface 306. An object 304coming into contact with the surface 306 will partially or entirelyblock beams within the contact area. FIGS. 3A and 3B illustrate somephysical mechanisms for touch interactions, but other mechanisms canalso be used. For example, the touch interaction may be based on changesin polarization, scattering, or changes in propagation direction orpropagation angle (either vertically or horizontally).

For example, FIG. 3C illustrates a different mechanism based onpropagation angle. In this example, the optical beam is guided in awaveguide 302 via TIR. The optical beam hits the waveguide-air interfaceat a certain angle and is reflected back at the same angle. However, thetouch 304 changes the angle at which the optical beam is propagating. InFIG. 3C, the optical beam travels at a steeper angle of propagationafter the touch 304. The detector D has a response that varies as afunction of the angle of propagation. The detector D could be moresensitive to the optical beam travelling at the original angle ofpropagation or it could be less sensitive. Regardless, an optical beamthat is disturbed by a touch 304 will produce a different response atdetector D.

In FIGS. 3A-3C, the touching object was also the object that interactedwith the beam. This will be referred to as a direct interaction. In anindirect interaction, the touching object interacts with an intermediateobject, which interacts with the optical beam. FIG. 3D shows an examplethat uses intermediate blocking structures 308. Normally, thesestructures 308 do not block the beam. However, in FIG. 3D, object 304contacts the blocking structure 308, which causes it to partially orentirely block the optical beam. In FIG. 3D, the structures 308 areshown as discrete objects, but they do not have to be so.

In FIG. 3E, the intermediate structure 310 is a compressible, partiallytransmitting sheet. When there is no touch, the sheet attenuates thebeam by a certain amount. In FIG. 3E, the touch 304 compresses thesheet, thus changing the attenuation of the beam. For example, the upperpart of the sheet may be more opaque than the lower part, so thatcompression decreases the transmittance. Alternately, the sheet may havea certain density of scattering sites. Compression increases the densityin the contact area, since the same number of scattering sites occupiesa smaller volume, thus decreasing the transmittance. Analogous indirectapproaches can also be used for frustrated TIR. Note that this approachcould be used to measure contact pressure or touch velocity, based onthe degree or rate of compression.

The touch mechanism may also enhance transmission, instead of or inaddition to reducing transmission. For example, the touch interaction inFIG. 3E might increase the transmission instead of reducing it. Theupper part of the sheet may be more transparent than the lower part, sothat compression increases the transmittance.

FIG. 3F shows another example where the transmittance between an emitterand detector increases due to a touch interaction. FIG. 3F is a topview. Emitter Ea normally produces a beam that is received by detectorD1. When there is no touch interaction, Ta1=1 and Ta2=0. However, atouch interaction 304 blocks the beam from reaching detector D1 andscatters some of the blocked light to detector D2. Thus, detector D2receives more light from emitter Ea than it normally would. Accordingly,when there is a touch event 304, Ta1 decreases and Ta2 increases.

For simplicity, in the remainder of this description, the touchmechanism will be assumed to be primarily of a blocking nature, meaningthat a beam from an emitter to a detector will be partially or fullyblocked by an intervening touch event. This is not required, but it isconvenient to illustrate various concepts.

For convenience, the touch interaction mechanism may sometimes beclassified as either binary or analog. A binary interaction is one thatbasically has two possible responses as a function of the touch.Examples includes non-blocking and fully blocking, or non-blocking and10%+ attenuation, or not frustrated and frustrated TIR. An analoginteraction is one that has a “grayscale” response to the touch:non-blocking passing through gradations of partially blocking toblocking. Whether the touch interaction mechanism is binary or analogdepends in part on the nature of the interaction between the touch andthe beam. It does not depend on the lateral width of the beam (which canalso be manipulated to obtain a binary or analog attenuation, asdescribed below), although it might depend on the vertical size of thebeam.

FIG. 4 is a graph illustrating a binary touch interaction mechanismcompared to an analog touch interaction mechanism. FIG. 4 graphs thetransmittance Tjk as a function of the depth z of the touch. Thedimension z is into and out of the active surface. Curve 410 is a binaryresponse. At low z (i.e., when the touch has not yet disturbed thebeam), the transmittance Tjk is at its maximum. However, at some pointz₀, the touch breaks the beam and the transmittance Tjk falls fairlysuddenly to its minimum value. Curve 420 shows an analog response wherethe transition from maximum Tjk to minimum Tjk occurs over a wider rangeof z. If curve 420 is well behaved, it is possible to estimate z fromthe measured value of Tjk.

C. Emitters, Detectors and Couplers

Each emitter transmits light to a number of detectors. Usually, eachemitter outputs light to more than one detector simultaneously.Similarly, each detector receives light from a number of differentemitters. The optical beams may be visible, infrared and/or ultravioletlight. The term “light” is meant to include all of these wavelengths andterms such as “optical” are to be interpreted accordingly.

Examples of the optical sources for the emitters include light emittingdiodes (LEDs) and semiconductor lasers. IR sources can also be used.Modulation of optical beams can be achieved by directly modulating theoptical source or by using an external modulator, for example a liquidcrystal modulator or a deflected mirror modulator. Examples of sensorelements for the detector include charge coupled devices, photodiodes,photoresistors, phototransistors, and nonlinear all-optical detectors.Typically, the detectors output an electrical signal that is a functionof the intensity of the received optical beam.

The emitters and detectors may also include optics and/or electronics inaddition to the main optical source and sensor element. For example,optics can be used to couple between the emitter/detector and thedesired beam path. Optics can also reshape or otherwise condition thebeam produced by the emitter or accepted by the detector. These opticsmay include lenses, Fresnel lenses, mirrors, filters, non-imaging opticsand other optical components.

In this disclosure, the optical paths will be shown unfolded forclarity. Thus, sources, optical beams and sensors will be shown as lyingin one plane. In actual implementations, the sources and sensorstypically will not lie in the same plane as the optical beams. Variouscoupling approaches can be used. A planar waveguide or optical fiber maybe used to couple light to/from the actual beam path. Free spacecoupling (e.g., lenses and mirrors) may also be used. A combination mayalso be used, for example waveguided along one dimension and free spacealong the other dimension. Various coupler designs are described in U.S.Application Ser. No. 61/510,989 “Optical Coupler” filed on Jul. 22,2011, which is incorporated by reference in its entirety herein.

D. Optical Beam Paths

Another aspect of a touch-sensitive system is the shape and location ofthe optical beams and beam paths. In FIGS. 1-2, the optical beams areshown as lines. These lines should be interpreted as representative ofthe beams, but the beams themselves are not necessarily narrow pencilbeams. FIGS. 5A-5C illustrate different beam shapes.

FIG. 5A shows a point emitter E, point detector D and a narrow “pencil”beam 510 from the emitter to the detector. In FIG. 5B, a point emitter Eproduces a fan-shaped beam 520 received by the wide detector D. In FIG.5C, a wide emitter E produces a “rectangular” beam 530 received by thewide detector D. These are top views of the beams and the shapes shownare the footprints of the beam paths. Thus, beam 510 has a line-likefootprint, beam 520 has a triangular footprint which is narrow at theemitter and wide at the detector, and beam 530 has a fairly constantwidth rectangular footprint. In FIG. 5, the detectors and emitters arerepresented by their widths, as seen by the beam path. The actualoptical sources and sensors may not be so wide. Rather, optics (e.g.,cylindrical lenses or mirrors) can be used to effectively widen ornarrow the lateral extent of the actual sources and sensors.

FIGS. 6A-6B and 7 show how the width of the footprint can determinewhether the transmission coefficient Tjk behaves as a binary or analogquantity. In these figures, a touch point has contact area 610. Assumethat the touch is fully blocking, so that any light that hits contactarea 610 will be blocked. FIG. 6A shows what happens as the touch pointmoves left to right past a narrow beam. In the leftmost situation, thebeam is not blocked at all (i.e., maximum Tjk) until the right edge ofthe contact area 610 interrupts the beam. At this point, the beam isfully blocked (i.e., minimum Tjk), as is also the case in the middlescenario. It continues as fully blocked until the entire contact areamoves through the beam. Then, the beam is again fully unblocked, asshown in the righthand scenario. Curve 710 in FIG. 7 shows thetransmittance Tjk as a function of the lateral position x of the contactarea 610. The sharp transitions between minimum and maximum Tjk show thebinary nature of this response.

FIG. 6B shows what happens as the touch point moves left to right past awide beam. In the leftmost scenario, the beam is just starting to beblocked. The transmittance Tjk starts to fall off but is at some valuebetween the minimum and maximum values. The transmittance Tjk continuesto fall as the touch point blocks more of the beam, until the middlesituation where the beam is fully blocked. Then the transmittance Tjkstarts to increase again as the contact area exits the beam, as shown inthe righthand situation. Curve 720 in FIG. 7 shows the transmittance Tjkas a function of the lateral position x of the contact area 610. Thetransition over a broad range of x shows the analog nature of thisresponse.

FIGS. 5-7 consider an individual beam path. In most implementations,each emitter and each detector will support multiple beam paths.

FIG. 8A is a top view illustrating the beam pattern produced by a pointemitter. Emitter Ej transmits beams to wide detectors D1-DK. Three beamsare shaded for clarity: beam j1, beam j(K−1) and an intermediate beam.Each beam has a fan-shaped footprint. The aggregate of all footprints isemitter Ej's coverage area. That is, any touch event that falls withinemitter Ej's coverage area will disturb at least one of the beams fromemitter Ej. FIG. 8B is a similar diagram, except that emitter Ej is awide emitter and produces beams with “rectangular” footprints (actually,trapezoidal but we will refer to them as rectangular). The three shadedbeams are for the same detectors as in FIG. 8A.

Note that every emitter Ej may not produce beams for every detector Dk.In FIG. 1, consider beam path aK which would go from emitter Ea todetector DK. First, the light produced by emitter Ea may not travel inthis direction (i.e., the radiant angle of the emitter may not be wideenough) so there may be no physical beam at all, or the acceptance angleof the detector may not be wide enough so that the detector does notdetect the incident light. Second, even if there was a beam and it wasdetectable, it may be ignored because the beam path is not located in aposition to produce useful information. Hence, the transmissioncoefficients Tjk may not have values for all combinations of emitters Ejand detectors Dk.

The footprints of individual beams from an emitter and the coverage areaof all beams from an emitter can be described using differentquantities. Spatial extent (i.e., width), angular extent (i.e., radiantangle for emitters, acceptance angle for detectors) and footprint shapeare quantities that can be used to describe individual beam paths aswell as an individual emitter's coverage area.

An individual beam path from one emitter Ej to one detector Dk can bedescribed by the emitter Ej's width, the detector Dk's width and/or theangles and shape defining the beam path between the two.

These individual beam paths can be aggregated over all detectors for oneemitter Ej to produce the coverage area for emitter Ej. Emitter Ej'scoverage area can be described by the emitter Ej's width, the aggregatewidth of the relevant detectors Dk and/or the angles and shape definingthe aggregate of the beam paths from emitter Ej. Note that theindividual footprints may overlap (see FIG. 8B close to the emitter).Therefore an emitter's coverage area may not be equal to the sum of itsfootprints. The ratio of (the sum of an emitter's footprints)/(emitter'scover area) is one measure of the amount of overlap.

The coverage areas for individual emitters can be aggregated over allemitters to obtain the overall coverage for the system. In this case,the shape of the overall coverage area is not so interesting because itshould cover the entirety of the active area 131. However, not allpoints within the active area 131 will be covered equally. Some pointsmay be traversed by many beam paths while other points traversed by farfewer. The distribution of beam paths over the active area 131 may becharacterized by calculating how many beam paths traverse different(x,y) points within the active area. The orientation of beam paths isanother aspect of the distribution. An (x,y) point that is derived fromthree beam paths that are all running roughly in the same directionusually will be a weaker distribution than a point that is traversed bythree beam paths that all run at 60 degree angles to each other.

The discussion above for emitters also holds for detectors. The diagramsconstructed for emitters in FIGS. 8A-8B can also be constructed fordetectors. For example, FIG. 8C shows a similar diagram for detector D1of FIG. 8B. That is, FIG. 8C shows all beam paths received by detectorD1. Note that in this example, the beam paths to detector D1 are onlyfrom emitters along the bottom edge of the active area. The emitters onthe left edge are not worth connecting to D1 and there are no emitterson the right edge (in this example design). FIG. 8D shows a diagram fordetector Dk, which is an analogous position to emitter Ej in FIG. 8B.

A detector Dk's coverage area is then the aggregate of all footprintsfor beams received by a detector Dk. The aggregate of all detectorcoverage areas gives the overall system coverage.

E. Active Area Coverage

The coverage of the active area 131 depends on the shapes of the beampaths, but also depends on the arrangement of emitters and detectors. Inmost applications, the active area is rectangular in shape, and theemitters and detectors are located along the four edges of therectangle.

In a preferred approach, rather than having only emitters along certainedges and only detectors along the other edges, emitters and detectorsare interleaved along the edges. FIG. 8E shows an example of this whereemitters and detectors are alternated along all four edges. The shadedbeams show the coverage area for emitter Ej.

F. Multiplexing

Since multiple emitters transmit multiple optical beams to multipledetectors, and since the behavior of individual beams is generallydesired, a multiplexing/demultiplexing scheme is used. For example, eachdetector typically outputs a single electrical signal indicative of theintensity of the incident light, regardless of whether that light isfrom one optical beam produced by one emitter or from many optical beamsproduced by many emitters. However, the transmittance Tjk is acharacteristic of an individual optical beam jk.

Different types of multiplexing can be used. Depending upon themultiplexing scheme used, the transmission characteristics of beams,including their content and when they are transmitted, may vary.Consequently, the choice of multiplexing scheme may affect both thephysical construction of the optical touch-sensitive device as well asits operation.

One approach is based on code division multiplexing. In this approach,the optical beams produced by each emitter are encoded using differentcodes. A detector receives an optical signal which is the combination ofoptical beams from different emitters, but the received beam can beseparated into its components based on the codes. This is described infurther detail in U.S. application Ser. No. 13/059,772 “Optical ControlSystem With Modulated Emitters,” which is incorporated by referenceherein.

Another similar approach is frequency division multiplexing. In thisapproach, rather than modulated by different codes, the optical beamsfrom different emitters are modulated by different frequencies. Thefrequencies are low enough that the different components in the detectedoptical beam can be recovered by electronic filtering or otherelectronic or software means.

Time division multiplexing can also be used. In this approach, differentemitters transmit beams at different times. The optical beams andtransmission coefficients Tjk are identified based on timing. If onlytime multiplexing is used, the controller must cycle through theemitters quickly enough to meet the required touch sampling rate.

Other multiplexing techniques commonly used with optical systems includewavelength division multiplexing, polarization multiplexing, spatialmultiplexing and angle multiplexing. Electronic modulation schemes, suchas PSK, QAM and OFDM, may also be possibly applied to distinguishdifferent beams.

Several multiplexing techniques may be used together. For example, timedivision multiplexing and code division multiplexing could be combined.Rather than code division multiplexing 128 emitters or time divisionmultiplexing 128 emitters, the emitters might be broken down into 8groups of 16. The 8 groups are time division multiplexed so that only 16emitters are operating at any one time, and those 16 emitters are codedivision multiplexed. This might be advantageous, for example, tominimize the number of emitters active at any given point in time toreduce the power requirements of the device.

III. Processing Phase

In the processing phase 220 of FIG. 2, the transmission coefficients Tjkare used to determine the locations of touch points. Differentapproaches and techniques can be used, including candidate touch points,line imaging, location interpolation, touch event templates, multi-passprocessing and beam weighting.

A. Candidate Touch Points

One approach to determine the location of touch points is based onidentifying beams that have been affected by a touch event (based on thetransmission coefficients Tjk) and then identifying intersections ofthese interrupted beams as candidate touch points. The list of candidatetouch points can be refined by considering other beams that are inproximity to the candidate touch points or by considering othercandidate touch points. This approach is described in further detail inU.S. patent application Ser. No. 13/059,817, “Method and Apparatus forDetecting a Multitouch Event in an Optical Touch-Sensitive Device,”which is incorporated herein by reference.

B. Line Imaging

This technique is based on the concept that the set of beams received bya detector form a line image of the touch points, where the viewpoint isthe detector's location. The detector functions as a one-dimensionalcamera that is looking at the collection of emitters. Due toreciprocity, the same is also true for emitters. The set of beamstransmitted by an emitter form a line image of the touch points, wherethe viewpoint is the emitter's location.

FIGS. 9-10 illustrate this concept using the emitter/detector layoutshown in FIGS. 8B-8D. For convenience, the term “beam terminal” will beused to refer to emitters and detectors. Thus, the set of beams from abeam terminal (which could be either an emitter or a detector) form aline image of the touch points, where the viewpoint is the beamterminal's location.

FIGS. 9A-C shows the physical set-up of active area, emitters anddetectors. In this example, there is a touch point with contact area910. FIG. 9A shows the beam pattern for beam terminal Dk, which are allthe beams from emitters Ej to detector Dk. A shaded emitter indicatesthat beam is interrupted, at least partially, by the touch point 910.FIG. 10A shows the corresponding line image 1021 “seen” by beam terminalDk. The beams to terminals Ea, Eb, . . . E(J−4) are uninterrupted so thetransmission coefficient is at full value. The touch point appears as aninterruption to the beams with beam terminals E(J−3), E(J−2) and E(J−1),with the main blockage for terminal E(J−2). That is, the portion of theline image spanning beam terminals E(J−3) to E(J−1) is a one-dimensionalimage of the touch event.

FIG. 9B shows the beam pattern for beam terminal D1 and FIG. 10B showsthe corresponding line image 1022 seen by beam terminal D1. Note thatthe line image does not span all emitters because the emitters on theleft edge of the active area do not form beam paths with detector D1.FIGS. 9C and 10C show the beam patterns and corresponding line image1023 seen by beam terminal Ej.

The example in FIGS. 9-10 use wide beam paths. However, the line imagetechnique may also be used with narrow or fan-shaped beam paths.

FIGS. 10A-C show different images of touch point 910. The location ofthe touch event can be determined by processing the line images. Forexample, approaches based on correlation or computerized tomographyalgorithms can be used to determine the location of the touch event 910.However, simpler approaches are preferred because they require lesscompute resources.

The touch point 910 casts a “shadow” in each of the lines images1021-1023. One approach is based on finding the edges of the shadow inthe line image and using the pixel values within the shadow to estimatethe center of the shadow. A line can then be drawn from a locationrepresenting the beam terminal to the center of the shadow. The touchpoint is assumed to lie along this line somewhere. That is, the line isa candidate line for positions of the touch point. FIG. 9D shows this.In FIG. 9D, line 920A is the candidate line corresponding to FIGS. 9Aand 10A. That is, it is the line from the center of detector Dk to thecenter of the shadow in line image 1021. Similarly, line 920B is thecandidate line corresponding to FIGS. 9B and 10B, and line 920C is theline corresponding to FIGS. 9C and 10C. The resulting candidate lines920A-C have one end fixed at the location of the beam terminal, with theangle of the candidate line interpolated from the shadow in the lineimage. The center of the touch event can be estimated by combining theintersections of these candidate lines.

Each line image shown in FIG. 10 was produced using the beam patternfrom a single beam terminal to all of the corresponding complimentarybeam terminals (i.e., beam pattern from one detector to allcorresponding emitters, or from one emitter to all correspondingdetectors). As another variation, the line images could be produced bycombining information from beam patterns of more than one beam terminal.FIG. 8E shows the beam pattern for emitter Ej. However, thecorresponding line image will have gaps because the correspondingdetectors do not provide continuous coverage. They are interleaved withemitters. However, the beam pattern for the adjacent detector Djproduces a line image that roughly fills in these gaps. Thus, the twopartial line images from emitter Ej and detector Dj can be combined toproduce a complete line image.

C. Location Interpolation

Applications typically will require a certain level of accuracy inlocating touch points. One approach to increase accuracy is to increasethe density of emitters, detectors and beam paths so that a small changein the location of the touch point will interrupt different beams.

Another approach is to interpolate between beams. In the line images ofFIGS. 10A-C, the touch point interrupts several beams but theinterruption has an analog response due to the beam width. Therefore,although the beam terminals may have a spacing of A, the location of thetouch point can be determined with greater accuracy by interpolatingbased on the analog values. This is also shown in curve 720 of FIG. 7.The measured Tjk can be used to interpolate the x position.

FIGS. 11A-B show one approach based on interpolation between adjacentbeam paths. FIG. 11A shows two beam paths a2 and b1. Both of these beampaths are wide and they are adjacent to each other. In all three casesshown in FIG. 11A, the touch point 1110 interrupts both beams. However,in the lefthand scenario, the touch point is mostly interrupting beama2. In the middle case, both beams are interrupted equally. In therighthand case, the touch point is mostly interrupting beam b1.

FIG. 11B graphs these two transmission coefficients as a function of x.Curve 1121 is for coefficient Ta2 and curve 1122 is for coefficient Tb1.By considering the two transmission coefficients Ta2 and Tb1, the xlocation of the touch point can be interpolated. For example, theinterpolation can be based on the difference or ratio of the twocoefficients.

The interpolation accuracy can be enhanced by accounting for any unevendistribution of light across the beams a2 and b1. For example, if thebeam cross section is Gaussian, this can be taken into account whenmaking the interpolation. In another variation, if the wide emitters anddetectors are themselves composed of several emitting or detectingunits, these can be decomposed into the individual elements to determinemore accurately the touch location. This may be done as a secondarypass, having first determined that there is touch activity in a givenlocation with a first pass. A wide emitter can be approximated bydriving several adjacent emitters simultaneously. A wide detector can beapproximated by combining the outputs of several detectors to form asingle signal.

FIG. 11C shows a situation where a large number of narrow beams is usedrather than interpolating a fewer number of wide beams. In this example,each beam is a pencil beam represented by a line in FIG. 11C. As thetouch point 1110 moves left to right, it interrupts different beams.Much of the resolution in determining the location of the touch point1110 is achieved by the fine spacing of the beam terminals. The edgebeams may be interpolated to provide an even finer location estimate.

D. Touch Event Templates

If the locations and shapes of the beam paths are known, which istypically the case for systems with fixed emitters, detectors andoptics, it is possible to predict in advance the transmissioncoefficients for a given touch event. Templates can be generated apriori for expected touch events. The determination of touch events thenbecomes a template matching problem.

If a brute force approach is used, then one template can be generatedfor each possible touch event. However, this can result in a largenumber of templates. For example, assume that one class of touch eventsis modeled as oval contact areas and assume that the beams are pencilbeams that are either fully blocked or fully unblocked. This class oftouch events can be parameterized as a function of five dimensions:length of major axis, length of minor axis, orientation of major axis, xlocation within the active area and y location within the active area. Abrute force exhaustive set of templates covering this class of touchevents must span these five dimensions. In addition, the template itselfmay have a large number of elements. Thus, it is desirable to simplifythe set of templates.

FIG. 12A shows all of the possible pencil beam paths between any two of30 beam terminals. In this example, beam terminals are not labeled asemitter or detector. Assume that there are sufficient emitters anddetectors to realize any of the possible beam paths. One possibletemplate for contact area 1210 is the set of all beam paths that wouldbe affected by the touch. However, this is a large number of beam paths,so template matching will be more difficult. In addition, this templateis very specific to contact area 1210. If the contact area changesslightly in size, shape or position, the template for contact area 1210will no longer match exactly. Also, if additional touches are presentelsewhere in the active area, the template will not match the detecteddata well. Thus, although using all possible beam paths can produce afairly discriminating template, it can also be computationally intensiveto implement.

FIG. 12B shows a simpler template based on only four beams that would beinterrupted by contact area 1210. This is a less specific template sinceother contact areas of slightly different shape, size or location willstill match this template. This is good in the sense that fewertemplates will be required to cover the space of possible contact areas.This template is less precise than the full template based on allinterrupted beams. However, it is also faster to match due to thesmaller size. These types of templates often are sparse relative to thefull set of possible transmission coefficients.

Note that a series of templates could be defined for contact area 1210,increasing in the number of beams contained in the template: a 2-beamtemplate, a 4-beam template, etc. In one embodiment, the beams that areinterrupted by contact area 1210 are ordered sequentially from 1 to N.An n-beam template can then be constructed by selecting the first nbeams in the order. Generally speaking, beams that are spatially orangularly diverse tend to yield better templates. That is, a templatewith three beams running at 60 degrees to each other and notintersecting at a common point tends to produce a more robust templatethan one based on three largely parallel beams which are in closeproximity to each other. In addition, more beams tends to increase theeffective signal-to-noise ratio of the template matching, particularlyif the beams are from different emitters and detectors.

The template in FIG. 12B can also be used to generate a family ofsimilar templates. In FIG. 12C, the contact area 1220 is the same as inFIG. 12B, but shifted to the right. The corresponding four-beam templatecan be generated by shifting beams (1,21) (2,23) and (3,24) in FIG. 12Bto the right to beams (4,18) (5,20) and (6,21), as shown in FIG. 12C.These types of templates can be abstracted. The abstraction will bereferred to as a template model. This particular model is defined by thebeams (12,28) (i, 22−i) (i+1,24−i) (i+2,25−i) for i=1 to 6. In oneapproach, the model is used to generate the individual templates and theactual data is matched against each of the individual templates. Inanother approach, the data is matched against the template model. Thematching process then includes determining whether there is a matchagainst the template model and, if so, which value of i produces thematch.

FIG. 12D shows a template that uses a “touch-free” zone around thecontact area. The actual contact area is 1230. However, it is assumedthat if contact is made in area 1230, then there will be no contact inthe immediately surrounding shaded area. Thus, the template includesboth (a) beams in the contact area 1230 that are interrupted, and (b)beams in the shaded area that are not interrupted. In FIG. 12D, thesolid lines (2,20) (5,22) and (11,27) are interrupted beams in thetemplate and the dashed lines (4,23) and (13,29) are uninterrupted beamsin the template. Note that the uninterrupted beams in the template maybe interrupted somewhere else by another touch point, so their useshould take this into consideration. For example, dashed beam (13,29)could be interrupted by touch point 1240.

FIG. 12E shows an example template that is based both on reduced andenhanced transmission coefficients. The solid lines (2,20) (5,22) and(11,27) are interrupted beams in the template, meaning that theirtransmission coefficients should decrease. However, the dashed line(18,24) is a beam for which the transmission coefficient should increasedue to reflection or scattering from the touch point 1250.

Other templates will be apparent and templates can be processed in anumber of ways. In a straightforward approach, the disturbances for thebeams in a template are simply summed or averaged. This can increase theoverall SNR for such a measurement, because each beam adds additionalsignal while the noise from each beam is presumably independent. Inanother approach, the sum or other combination could be a weightedprocess, where not all beams in the template are given equal weight. Forexample, the beams which pass close to the center of the touch eventbeing modeled could be weighted more heavily than those that are furtheraway. Alternately, the angular diversity of beams in the template couldalso be expressed by weighting. Angular diverse beams are more heavilyweighted than beams that are not as diverse.

In a case where there is a series of N beams, the analysis can beginwith a relatively small number of beams. Additional beams can be addedto the processing as needed until a certain confidence level (or SNR) isreached. The selection of which beams should be added next could proceedaccording to a predetermined schedule. Alternately, it could proceeddepending on the processing results up to that time. For example, ifbeams with a certain orientation are giving low confidence results, morebeams along that orientation may be added (at the expense of beams alongother orientations) in order to increase the overall confidence.

The data records for templates can also include additional details aboutthe template. This information may include, for example, location of thecontact area, size and shape of the contact area and the type of touchevent being modeled (e.g., finger, stylus, etc.).

In addition to intelligent design and selection of templates, symmetriescan also be used to reduce the number of templates and/or computationalload. Many applications use a rectangular active area with emitters anddetectors placed symmetrically with respect to x and y axes. In thatcase, quadrant symmetry can be used to achieve a factor of fourreduction. Templates created for one quadrant can be extended to theother three quadrants by taking advantage of the symmetry. Alternately,data for possible touch points in the other three quadrants can betransformed and then matched against templates from a single quadrant.If the active area is square, then there may be eight-fold symmetry.

Other types of redundancies, such as shift-invariance, can also reducethe number of templates and/or computational load. The template model ofFIGS. 12B-C is one example.

In addition, the order of processing templates can also be used toreduce the computational load. There can be substantial similaritiesbetween the templates for touches which are nearby. They may have manybeams in common, for example. This can be taken advantage of byadvancing through the templates in an order that allows one to takeadvantage of the processing of the previous templates.

E. Multi-Pass Processing

Referring to FIG. 2, the processing phase need not be a single-passprocess nor is it limited to a single technique. Multiple processingtechniques may be combined or otherwise used together to determine thelocations of touch events.

FIG. 13 is a flow diagram of a multi-pass processing phase based onseveral stages. This example uses the physical set-up shown in FIG. 9,where wide beams are transmitted from emitters to detectors. Thetransmission coefficients Tjk are analog values, ranging from 0 (fullyblocked) to 1 (fully unblocked).

The first stage 1310 is a coarse pass that relies on a fast binarytemplate matching, as described with respect to FIGS. 12B-D. In thisstage, the templates are binary and the transmittances T′jk are alsoassumed to be binary. The binary transmittances T′jk can be generatedfrom the analog values Tjk by rounding or thresholding 1312 the analogvalues. The binary values T′jk are matched 1314 against binary templatesto produce a preliminary list of candidate touch points. Thresholdingtransmittance values may be problematic if some types of touches do notgenerate any beams over the threshold value. An alternative is tothreshold the combination (by summation for example) of individualtransmittance values.

Some simple clean-up 1316 is performed to refine this list. For example,it may be simple to eliminate redundant candidate touch points or tocombine candidate touch points that are close or similar to each other.For example, the binary transmittances T′jk might match the template fora 5 mm diameter touch at location (x,y), a 7 mm diameter touch at (x,y)and a 9 mm diameter touch at (x,y). These may be consolidated into asingle candidate touch point at location (x,y).

Stage 1320 is used to eliminate false positives, using a more refinedapproach. For each candidate touch point, neighboring beams may be usedto validate or eliminate the candidate as an actual touch point. Thetechniques described in U.S. patent application Ser. No. 13/059,817 maybe used for this purpose. This stage may also use the analog values Tjk,in addition to accounting for the actual width of the optical beams. Theoutput of stage 1320 is a list of confirmed touch points.

The final stage 1330 refines the location of each touch point. Forexample, the interpolation techniques described previously can be usedto determine the locations with better accuracy. Since the approximatelocation is already known, stage 1330 may work with a much smallernumber of beams (i.e., those in the local vicinity) but might apply moreintensive computations to that data. The end result is a determinationof the touch locations.

Other techniques may also be used for multi-pass processing. Forexample, line images or touch event models may also be used.Alternatively, the same technique may be used more than once or in aniterative fashion. For example, low resolution templates may be usedfirst to determine a set of candidate touch locations, and then higherresolution templates or touch event models may be used to more preciselydetermine the precise location and shape of the touch.

F. Beam Weighting

In processing the transmission coefficients, it is common to weight orto prioritize the transmission coefficients. Weighting effectively meansthat some beams are more important than others. Weightings may bedetermined during processing as needed, or they may be predetermined andretrieved from lookup tables or lists.

One factor for weighting beams is angular diversity. Usually, angularlydiverse beams are given a higher weight than beams with comparativelyless angular diversity. Given one beam, a second beam with small angulardiversity (i.e., roughly parallel to the first beam) may be weightedlower because it provides relatively little additional information aboutthe location of the touch event beyond what the first beam provides.Conversely, a second beam which has a high angular diversity relative tothe first beam may be given a higher weight in determining where alongthe first beam the touch point occurs.

Another factor for weighting beams is position difference between theemitters and/or detectors of the beams (i.e., spatial diversity).Usually, greater spatial diversity is given a higher weight since itrepresents “more” information compared to what is already available.

Another possible factor for weighting beams is the density of beams. Ifthere are many beams traversing a region of the active area, then eachbeam is just one of many and any individual beam is less important andmay be weighted less. Conversely, if there are few beams traversing aregion of the active area, then each of those beams is more significantin the information that it carries and may be weighted more.

In another aspect, the nominal beam transmittance (i.e., thetransmittance in the absence of a touch event) could be used to weightbeams. Beams with higher nominal transmittance can be considered to bemore “trustworthy” than those which have lower nominal transmittancesince those are more vulnerable to noise. A signal-to-noise ratio, ifavailable, can be used in a similar fashion to weight beams. Beams withhigher signal-to-noise ratio may be considered to be more “trustworthy”and given higher weight.

The weightings, however determined, can be used in the calculation of afigure of merit (confidence) of a given template associated with apossible touch location. Beam transmittance/signal-to-noise ratio canalso be used in the interpolation process, being gathered into a singlemeasurement of confidence associated with the interpolated line derivedfrom a given touch shadow in a line image. Those interpolated lineswhich are derived from a shadow composed of “trustworthy” beams can begiven greater weight in the determination of the final touch pointlocation than those which are derived from dubious beam data.

These weightings can be used in a number of different ways. In oneapproach, whether a candidate touch point is an actual touch event isdetermined based on combining the transmission coefficients for thebeams (or a subset of the beams) that would be disturbed by thecandidate touch point. The transmission coefficients can be combined indifferent ways: summing, averaging, taking median/percentile values ortaking the root mean square, for example. The weightings can be includedas part of this process: taking a weighted average rather than anunweighted average, for example. Combining multiple beams that overlapwith a common contact area can result in a higher signal to noise ratioand/or a greater confidence decision. The combining can also beperformed incrementally or iteratively, increasing the number of beamscombined as necessary to achieve higher SNR, higher confidence decisionand/or to otherwise reduce ambiguities in the determination of touchevents.

IV. Applications

The touch-sensitive devices described above can be used in variousapplications. Touch-sensitive displays are one class of application.This includes displays for tablets, laptops, desktops, gaming consoles,smart phones and other types of compute devices. It also includesdisplays for TVs, digital signage, public information, whiteboards,e-readers and other types of good resolution displays. However, they canalso be used on smaller or lower resolution displays: simpler cellphones, user controls (photocopier controls, printer controls, controlof appliances, etc.). These touch-sensitive devices can also be used inapplications other than displays. The “surface” over which the touchesare detected could be a passive element, such as a printed image orsimply some hard surface. This application could be used as a userinterface, similar to a trackball or mouse.

V. Additional Considerations

The figures depict embodiments of the present invention for purposes ofillustration only. One skilled in the art will readily recognize fromthe following discussion that alternative embodiments of the structuresand methods illustrated herein may be employed without departing fromthe principles of the invention described herein.

Upon reading this disclosure, those of skill in the art will appreciatestill additional alternative structural and functional designs throughthe disclosed principles herein. Thus, while particular embodiments andapplications have been illustrated and described, it is to be understoodthat the disclosed embodiments are not limited to the preciseconstruction and components disclosed herein. Various modifications,changes and variations, which will be apparent to those skilled in theart, may be made in the arrangement, operation and details of the methodand apparatus disclosed herein without departing from the spirit andscope defined in the appended claims.

What is claimed is:
 1. A method for determining simultaneous touchevents on a surface, the surface having at least one emitter and aplurality of detectors arranged around its periphery, the at least oneemitter producing a plurality of optical beams received by thedetectors, the touch events disturbing the optical beams, the methodcomprising: determining, a priori, a set of touch event templates, eachtouch event template including a size of a contact area and a set ofbeams that would be disturbed by a touch event of the size of thecontact area; receiving information indicating which beams have beendisturbed by at least one touch event; comparing the receivedinformation indicating which beams have been disturbed to the sets ofbeams identified in the touch event templates; determining anapproximate location and size of a contact area of at least one touchevent on the surface based on a match between the received informationand a given touch event template; and refining the approximate locationof the at least one touch event to determine a precise location of theat least one touch event.
 2. The method of claim 1, wherein the sizes ofthe contact areas of the templates are based on a major axis length anda minor axis length.
 3. The method of claim 1, wherein the set of eachtouch event template includes at least two beams.
 4. The method of claim1 wherein the received information includes transmission coefficients,each transmission coefficient indicative that a transmission of one ofthe beams between an emitter and a detector has been disturbed.
 5. Themethod of claim 4, wherein refining the approximate location of the atleast one touch event to determine the precise location of the at leastone touch event comprises: analyzing the transmission coefficients todetermine the precise location of the at least one touch event.
 6. Themethod of claim 5, wherein analyzing the transmission coefficients todetermine the precise location of the at least one touch eventcomprises: interpolating the precise location based on the transmissioncoefficients.
 7. The method of claim 6, wherein the interpolation usesat least one of a difference between transmission coefficients and aratio of transmission coefficients.
 8. The method of claim 1, whereinrefining the approximate location of the at least one touch event todetermine the precise location of the at least one touch eventcomprises: analyzing a subset of the set of the beams associated withone of the touch event templates used to determine the at least onetouch event and which were disturbed by the at least one touch event. 9.The method of claim 1, wherein refining the approximate location of theat least one touch event to determine the precise location of the atleast one touch event is based on a beam distribution of the at leastone emitter associated with the beams disturbed by the at least onetouch event.
 10. The method of claim 1 wherein, within a touch eventtemplate, at least one of no two beams are parallel, and no two beamsare sent to the same detector.
 11. The method of claim 1 wherein thenumber of touch event templates in the set of touch event templates isreduced due to symmetry.
 12. A touch sensitive device comprising: atleast one emitter and a plurality of detectors arranged around aperiphery of a surface, the at least one emitter producing a pluralityof optical beams received by the detectors, the touch events disturbingthe optical beams; a processor; a non-transitory computer readablestorage medium comprising computer program instructions that whenexecuted by the processor cause the processor to: determine, a priori, aset of touch event templates, each touch event template including a sizeof a contact area and a set of beams that would be disturbed by a touchevent of the size of the contact area; receive information indicatingwhich beams have been disturbed by at least one touch event; compare thereceived information indicating which beams have been disturbed to thesets of beams identified in the touch event templates; determine anapproximate location and size of a contact area of at least one touchevent on the surface based on a match between the received informationand a given touch event template; and refine the approximate location ofthe at least one touch event to determine a precise location of the atleast one touch event.
 13. The device of claim 12, wherein the expectedsizes of the contact areas of the templates are based on a major axislength and a minor axis length.
 14. The device of claim 12, wherein theset of each touch event template includes at least two beams.
 15. Thedevice of claim 12 wherein the received information includestransmission coefficients, each transmission coefficient indicative thata transmission of one of the beams between an emitter and a detector hasbeen disturbed.
 16. The device of claim 15, wherein refining theapproximate location of the at least one touch event to determine theprecise location of the at least one touch event comprises: analyzingthe transmission coefficients to determine the precise location of theat least one touch event.
 17. The device of claim 16, wherein analyzingthe transmission coefficients to determine the precise location of theat least one touch event comprises: interpolating the precise locationbased on the transmission coefficients.
 18. The device of claim 17,wherein the interpolation uses at least one of a difference betweentransmission coefficients and a ratio of transmission coefficients. 19.The device of claim 12, wherein refining the approximate location of theat least one touch event to determine the precise location of the atleast one touch event comprises: analyzing a subset of the set of thebeams associated with one of the touch event templates used to determinethe at least one touch event and which were disturbed by the at leastone touch event.
 20. The device of claim 12, wherein refining theapproximate location of the at least one touch event to determine theprecise location of the at least one touch event is based on a beamdistribution of the at least one emitter associated with the beamsdisturbed by the actual touch event.
 21. The device of claim 12 wherein,within a touch event template, at least one of no two beams areparallel, and no two beams are sent to the same detector.
 22. The deviceof claim 12 wherein the number of touch event templates in the set oftouch event templates is reduced due to symmetry.